1. Field of the Invention
The present invention relates to a charged particle beam apparatus, such as an electron microscope, an electron beam writer, a focused ion beam system, etc. and in particular, to an evacuating technology that can achieve extremely high degree of vacuum of an electron gun or an ion gun.
2. Description of the Related Art
A scanning electron microscope (SEM) or an electron beam writer (EB) according to the related art accelerates electron beams emitted from an electron gun including a cold cathode field emission type electron source or a thermal field emitter electron source, which are changed into fine electron beams using an electron lens, and scans them as primary electron beams on a sample by using a scanning deflector. In the case of SEM, images are obtained by detecting secondary electrons or reflected electrons and in the case of EB, patterns previously registered on a resist film that is applied on the sample are written. As a material for the field emission type electron source, tungsten has been used in the case of a multi-purpose SEM. Further, in the case of the EB, LaB6 may be used.
The cold cathode field emission type electron gun is a field emission type electron gun that uses a needle-shaped tungsten tip at normal temperature. The electrons are emitted by a tunnel effect that is generated by applying high field to a point of the tip. Brightness is ˜108, which is better than that of the thermal field (emission) type. An energy band of the emission electron is narrower than that of the thermal field type (˜0.4 eV) and high energy resolution can be obtained. A smaller probe compared with that of the thermal field type can be manufactured, but the total amount of emission current is smaller than that of the thermal field type. Meanwhile, the thermal field type (generally referred to as Schottky electron gun) is an electron gun according to a scheme that heats the needle-shaped tungsten tip under field and emits electrons. The thermal field emitter electron gun heats a tip to ˜1800 K and emits electrons using a Schottky effect, wherein the tungsten tip is coated with zirconia and a potential barrier of the tip is lowered (˜2.7 eV). The energy band of the emission electron is 0.7 eV, which is slightly wider than that of the cold cathode type, but since the tip is heated all time, stable emission current (variation ratio ˜1%) can be obtained without absorbing residual gases on a surface of the tip. Brightness is about 107 to 108.
In order to emit good electron beams from the electron source over a long period of time, there is a need to keep the surrounding area of the electron source at high vacuum (10−8 to 10−9 Pa). To this end, the related art has used a method that forcibly evacuates the surrounding area of the electron gun by an ion pump. Further, there is a charged particle beam apparatus that has a non-evaporable getter pump therein to achieve a higher degree of vacuum (For example, see U.S. Pat. No. 4,833,362 and Japanese Patent Application Laid-Open No. 2006-294481). This is particularly an effective method for the cold cathode field emission type electron source or the thermal field emitter electron source (Schottky electron source).
When a pumping speed is about 20 L/s, generally, the ion pump has a size of 15 cm to 20 cm, which is a pump including a high voltage electrode and a magnet. However, it is difficult to build the ion pump in the vicinity of the electron source. Therefore, the ion pump that is adhered to a side of a barrel (column) configured of the electron gun including the electron source and an electron optical system is usually used.
The non-evaporable getter pump is a pump that chemically absorbs and fixes gas molecules to a special alloy surface and if the surface of the pump is activated by being heated once, it can continue a pumping function without needing any energy. Because the alloy surface is covered with the gas molecules, the pumping speed is reduced, but if the surface of the pump is reactivated by being heated, the gas molecules absorbed on the surface are diffused into the alloy and permanently fixed thereto, such that the non-evaporable pump has a characteristic when a pure surface is exposed, the surface returns to a state capable of absorbing gases again.
Moreover, there is a charged particle beam apparatus including an electron source using a photo-cathode that is different from the field emission type electron source as described in Japanese Unexamined Patent Application Publication No. 2002-500809. If the electron source makes gallium, arsenic, etc., which are special materials, into a thin film and intensively irradiates a laser beam to the rear of the thin film, it also excites an irradiated the thin film to emit electrons. The electron source is suitable for applications for which the time-divided pulse-shaped electron beams are necessary. However, since a size of a light source is large or it is difficult to continuously emit electrons, the electron source is not suitable for observing high resolution which is an object of the present invention. In addition, it can be said that an electron emission mechanism of the photo-cathode uses different physics, and is a totally different technology.